Flow is the key to casting big long bullets. You need to get the alloy in the mold before it starts to solidify. I cast alot of long heavy bullets, in the 1.125- 1.300 range from 38 cal to 45 caliber. Watch our sprue puddle it should take 4-6 secs to "frost" over.
I cast these bullets with a ladle. Lyman or rcbs both have spouts opened to .205 dia from just under 3/16" much faster pour and better delivery. This modification greatly improved my results, with out raising temps or tin content.
I believe it is more a result of bullet length than weight causing this. As the longer bullets start to cool sooner in the pour than the shorter ones, and the heat is less localized in the blocks.
One difference in industry casting is those casting machines have a shot tube on them thats filled with the molten metal, then it pushes the material in under pressure filling the mould almost instantly, an option we dont have. The 'shot" goes in under several hundred psi and is held for a set time. This makes a mush better casting both in form and structure.
I would recommend opening the spout up some on the bottom pour to increase flow or a modified ladle a described above. Warm your mould up to temp and cast a fairly brisk pace adjusting pace as needed. Workin the 700*-725* range and adjust from there.